1. Field of the Invention
The present invention relates to a flexible laminate of films from thermoplastic polymer material mainly for applications in which relatively high yield strength and ultimate tensile strength is required, and a method and apparatus for its manufacture. In special aspect it also relates to film laminates which allow air but not water to penetrate, and laminates with properties as filter material. Examples of applications are: tarpaulins and cover-sheets, pond liners, substitute geotextiles, weather or gas protective garments, greenhouse film, industrial bags or garbage bags, carrier bags, self-standing pouches, and sanitary backsheets.
2. Description of the Related Art
For economical reasons there is an increased need to reduce the square meter weight of flexible film made from thermoplastic polymer material. The limits are partly set by the required strength properties, and partly by the required self supporting capability, i.e. stiffness or resilience with respect to bending. These needs have mainly been met by selected developments of the thermoplastic polymer compositions and as far as the strength is concerned also by biaxial orientation, or by cross lamination of films each of which exhibits a generally monoaxial or unbalanced biaxial unorientation. From the strength point of view essential savings can be achieved by such orientation and/or crosslamination processes.                Thus as an example an industrial bag made from extruded polyethylene film of the best suited grades and destined for packing of 25 kg polyethylene granules must generally have a thickness of 0.12-0.15 mm in order to satisfy the normal strength requirements, while this thickness can be brought down to about 0.07 mm by use of optimized oriented and crosslaminated film from polyethylene. However, when this crosslaminate is made in the known manner, few available types of machines for manufacturing bags from film, and few available types of machines for filling the bags, can work adequately with film which is so thin and flimsy.        
A crosslaminate which, besides the improved strength properties obtained by the orientation and crosslamination, also by virtue of its geometrical structure shows significant improvements in this respect, is described in the inventor's earlier specification EP-A-0624126.
This is a crosslaminate of a slightly waved configuration in which the material in the curved crests on one or both sides of the laminate is thicker than elsewhere, the material between these thicker curved crests being generally straightened out. (See FIGS. 14 and 15 of said patent publication). The structure is obtained by stretching between several sets of grooved rollers under special conditions. This stretching also imparts transverse orientation. The disclosed wavelengths of the final product are between 2.2 and 3.1 mm.
Crosslaminates according to EP-A-0624126 have been produced industrially since 1995 for manufacture of industrial bags from combinations of high molecular weight high density polyethylene (HMWHDPE) and linear low density polyethylene (LLDPE) with film weight about 90 gm−2. The slightly waved shape in combination with the thickened crests imparts a stiffness and resilience in one direction of the film which has proven to be very important for the performance of the bag machines with such relatively thin film. However a film of a similar structure, but with a 70gm−2 gauge, which satisfies the strength requirements is too flimsy for the making of bags.
As another example an agricultural tarpaulin (e.g. for protection of crops) made from a 70 gm crosslaminate of oriented polyethylene films would be a fully adequate substitute for a 100 gm−2 tarpaulin made from extrusion-coated woven tape, if only objective criteria were applied. However, the average customer of agricultural tarpaulins makes his choice to a great extent on the basis of the handle and the appearance, and will reject the 70 gm −2 tarpaulin due to its flimsiness, judging that it lacks 3 a substance.
The stiffness can of course always be increased by suitable incorporation of a filler, (and the present invention includes that as an additional option) but this will always more or less be at the expense of puncture and tear propagation resistance, especially under impact.